Roller cone or PDC type drill bits typically are turned about 50-300 RPM. When driven with a downhole motor a Moineau design is typically used to rotate the bit. This progressing cavity type of a motor features a rubber stator with a metallic rotor turning in it and the circulating fluid causes shaft rotation as the progressing cavity makes the rotor attached to the bit rotate at a speed determined by the motor configuration and the flowing fluid parameters. The issue with such downhole motors is a temperature service limit of about 380 degrees F. because of the use of the rubber components. Many well environments have higher temperature so that an alternative way is needed to drive the bit in those high temperature applications.
Turbines have been used in downhole applications that turn drill bits with a gearbox for the proper output speed for the bit. Such a design is illustrated in U.S. Pat. No. 4,434,862. Applications with gearboxes have similar high temperature issues for the gearbox seal materials and lubricant performance issues. Other references that use turbines in downhole applications are U.S. Pat. Nos. 4,678,045; 5,394,951; 5,517,464 (driving a generator); U.S. Pat. No. 7,140,444 (driving a rotary cutter) and U.S. Pat. No. 7,066,284 (turbine as a driver option for a bottom hole assembly of a bit and associated reamer.
Turbine applications in the past have either not been coupled to bits or if coupled to bits employed mechanical drives that had enclosed housings and required seals that had temperature service limits akin to the progressing cavity pumps that could rotate at the desired bit speed without any speed reduction.
Cycloidal speed reduction devices have been used in the automotive industry for differentials as illustrated in U.S. Pat. No. 7,749,123. The principal has been employed as a downhole motor design in U.S. Pat. No. 7,226,279 and as part of a rotary steerable bottom hole assembly in U.S. Pat. No. 7,467,673. A Cycloidal speed reducer of a known design is illustrated in FIG. 2. An input shaft 10 is connected to a motor or driver 12. The shaft 10 is connected to the hub 14 eccentrically. A gear 16 turns with hub 14 in an eccentric manner. The gear 16 has a series of external lobes 18. A stator 20 is held fixed around the lobes 18 and has gaps 22 into which the lobes 18 enter and exit as the gear 16 rotates eccentrically. The gear 16 has a series of holes 24 through which extend rods 26 connected to the shaft 28. As the gear 16 rotates eccentrically at a high speed, the rods 26 define a movement pattern that follows the circular edges of the holes 24. As a result the shaft 28 rotates in the opposite direction from the shaft 10 and at a slower speed. The reduction rate of the cycloidal drive is obtained from the following formula, where P means the number of the ring gear pins 30 and L is the number of pins 32 on the cycloidal disc.
  r  =            P      -      L        L  
The advantage of a cycloidal drive is that it is an open transmission system that is well suited to a high temperature application since it does not require temperature sensitive seals. Since turbines typically operate at speeds well above the typical rate of drill bits it makes the coupling of a turbine drive to avoid the temperature limitations of a progressing cavity Moineau pump well suited for the use of cycloidal gearing to get a suitable output speed for the bit. The turbine exhaust can also run through the speed reducer to allow greater design flexibility in component layout in a space constrained environment. While there are some issues with cycloidal speed reducers such as vibration there are simple solutions to those issues while keeping the overall design simple and compact. Those skilled in the art will more readily appreciate the present invention by a review of the description of the preferred embodiment and the associated drawing while recognizing that the full scope of the invention is to be found in the appended claims.